media-export: Don't delete file from fs event

If a file is removed by the user, then don't try to remove the file
again. There are some tools that move by remove & copy and Rygel will
then remove the just copied file.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=734303